Multilayer films consisting of various polymeric materials have many applications in packaging structures. Polymeric materials are used as films, sheets, lidstock, pouches, tubes and bags. One type of packaging structure for which polymeric materials are used is for the packaging of meats.
Generally, meats are packaged in bags made from heat shrinkable thermoplastic films known as shrink bags. After the film is produced, it is folded and the product is inserted. The air is then evacuated from the bag and the film is sealed along its edges, such as by heat sealing. Subsequently, the film is heated to initiate heat shrinkage about the meat.
In order to successfully serve as the material for a shrink bag, a thermoplastic film must be able to remain intact during the packaging process where it is filled, evacuated, sealed (as by heat-sealing), and heat shrunk. The heat sealing and heat shrinking operations require subjecting the film to high temperatures. Thus, the thermoplastic film must have sufficient strength at high temperatures known as “hot strength”. The thermoplastic film must also remain intact during the shipment and handling of the shrink-wrapped product as to physically protect the meat.
In addition to remaining intact during the fabrication of the bag and subsequent handling, the thermoplastic film must also serve as a barrier for the product from gaseous materials in the environment. The film must especially serve as a barrier to the infusion of oxygen, thus preventing spoilage of the product caused by the exposure to oxygen.
It is desirable that the thermoplastic film achieve the properties of a shrink bag while remaining economical to produce. Thus, the resin for the film should be relatively inexpensive, readily extrudable, and the film produced therefrom susceptible to orientation.
Shrink bag packaging of meat has several inherent difficulties, many of which are attributable to limitations in the thermoplastic films. For example, the process of stretching the film and shrinking it expose the film to severe stress conditions due to the nature of the operation. The thermoplastic films are especially vulnerable to failure at these severe conditions.
The thermoplastic film must also be capable of being oriented without distortion, and without delamination of the multiple layers normally present in packaging films. The thermoplastic film must be strong enough at the orientation temperature to withstand stretching without the creation of holes, tears, or non-uniform zones of stretching.
In the case of blown tubular thermoplastic films, the film must be capable of being stretched to form a bubble during the orientation process. Each of the layers of th film should be able to be oriented without fracture, separation, or creation of holes in the layer.
In packaging use, the thermoplastic film must respond to heat rapidly enough for commercial practicality, and yet must not exhibit such a level of shrink energy that would cause the film to pull apart or delaminate as a result of shrinkage from its own internal forces.
Film packages are known in the art to be susceptible to failure or deformation at any location where portions of the films are sealed to each other by heat sealing. In the formation of a heat seal, at least portions of the film are heated to a temperature where they are soft enough to flow and melt merge when subjected to pressure. It is desirable to be able to form heat seals in a film over a range of temperatures and pressures so that commercial processes can be adjusted within the normal operating parameters. Whatever the acceptable range of parameters for the formations of heat seals, it is critical that the seals have adequate strength to hold the package closed, and prevent leakage into or out of the package until it is intentionally opened. Thus, the strength of the heat seals is also an important measure of films used in applications such as shrink bags, where heat seals are formed.
A solution known in the art to the problem of failure or deformation at the heat seals is the cross-linking of the film layers by irradiation prior to heat sealing. Cross-linking the film provides improved toughness and increases the heat seal temperature range.
However, cross-linked thermoplastic films are more difficult to melt and provide weaker seals than unirradiated films when heat sealed. Users require that the seals maintain their integrity when the bag containing meat or other food article is immersed in hot water to shrink the film. A bag with weak heat seals that ruptures when the bag is shrunk, is of no use. Thus, there is a need for an irradiated multiple layer film which can be made into a bag that will have strong seals when heat sealed.
Another important measure of the value of film for use in shrink bag applications is optical clarity. A desirable film has good optical clarity, so that the consumer can view the product clearly and without obstructions. Thus, a desirable film should be free of haze or other imperfections.
U.S. Pat. No. 4,457,960, which is commonly owned with this application and hereby incorporated by reference, discloses multiple layer, molecularly oriented films, and packages made from the films, which may be used in shrink bag applications. The disclosed film contains a first barrier layer, preferably either polyvinylidene chloride copolymer (Saran) or ethylene vinyl alcohol (EVOH), and at least second and third layers, both of which are firmly adhered to the first barrier layer. The second layer comprises a blend of linear low density polyethylene (“LLDPE”) and EVA. The third layer comprises ethylene vinyl acetat (EVA) or blends of EVA and linear low density polyethylene.
U.S. Pat. No. 4,894,107, which is also commonly owned with this application and hereby incorporated by reference, discloses multiple layer, molecularly oriented films, and packages made from the films, which may be used in shrink bag applications. The films contain a core barrier layer of vinylidene chloride copolymer (VDC-CP) or vinylidene chloride methylacrylate (VDC-MA), as well as at least two additional layers comprising blends of LLDPE and EVA. U.S. Pat. No. 4,894,107 also teaches electron beam irradiation of the film in an amount of at least 1.5 megarads, preferably 2 to 5 megarads. The irradiation induces cross-linking, thereby improving the hot strength and heat seal properties of the film.
Pending U.S. patent application Ser. No. 08/011,528, which is commonly owned with this application and hereby incorporated by reference, discloses multiple layer films with inner heat sealant layers and outer protective layers that have different degrees of cross-linking at different levels of irradiation doses.